Combustion behaviour and emission performance of neat blended polyoxymethylene dimethyl ethers in a light-duty diesel engine

The combustion behaviour, the mechanisms of soot formation, and the emission performance of a mixture of polyoxymethylene dimethyl ethers (POMDME) oligomers with a number of oxymethylene units ranging from 3 to 5, both neat and blended at 12.5% and 50% levels with commercial diesel fuel have been investigated. The goals were a first evaluation of the POMDME impact on the diesel injector behaviour, on the combustion process as well as on the emission performance of a light duty engine. Then a brief screening on the capability to improve the NOx-PM trade-off using POMDME by means of the exhaust gas recirculation (EGR) rate increment was also assessed. Therefore, the experiments were carried out first using an injector test rig to control injection parameters, then in a transparent singlecylinder research engine with optical access for combustion visualization with spatially-resolved measurements of flame temperature and soot concentration, and finally in a light-duty multi-cylinder engine. Two operating conditions were chosen for the tests as representative of the most critical point for emission and noise optimization: engine speed of 1500 rpm and brake mean effective pressure (BMEP) of 5 bar, and 2500 rpm and 8 bar BMEP. Results from the single-cylinder engine indicate that the presence of the POMDME affects essentially the process of soot oxidation and results in a significant increase of the oxidation rate. This effect is attributed to the presence of intramolecular oxygen in the fuel which is readily available in situ and to the higher flame temperatures. The tests with the multi-cylinder engine have demonstrated that with the neat POMDME and to a lesser extent also with the 50% blend it is possible a simultaneous optimization of NOx and PM emissions and also of the noise level to a limit that could not be reached with conventional diesel fuels. For the 10-12% blend that could be used in nondedicated engines, the reduction of PM emission was about 40%, while with higher blends and EGR recalibration in dedicated vehicle fleets very low NOx exhaust emission can be reached.